1. Field of the Invention
The present invention relates to semiconductor circuits including transistors such as bipolar transistors or field-effect transistors. More specifically, the present invention relates to a cascode distributed amplifier in which a plurality of unit circuits, each including a grounded-gate transistor, are connected in parallel.
Recently, with the widespread proliferation of the Internet, the demand for a broadband transmission system capable of transmitting and receiving a large quantity of data at a high bit rate of 40 G b/s has increased. For this reason, there is also the demand for a broadband amplifier with a high frequency band of 40 GHz or above, which is one component of the broadband transmission system of the above type. In a transmitter of the broadband transmission system, a driver circuit is disposed at the front end the transmitter to drive a LN (lithium niobate) modulator. To obtain a high output voltage Vpp, ranging from 5 to 6 volts, and a good eye opening property, the driver circuit requires that it has a gain-raising characteristic to compensate for a loss of the LN modulator.
2. Description of the Related Art
It is known that a distributed amplifier provides a bandwidth depending on the arrangement of a ladder filter having the transistors with input capacitance and the transmission wires with inductance, and that the distributed amplifier is suitable for a broadband amplifier. In recent years, the major trend for such broadband amplifier is a cascode distributed amplifier in which a plurality of unit circuits each including a grounded-gate transistor are connected in parallel. The use of the cascode distributed amplifier allows generation of a negative-polarity resistance in a high frequency band by a circuit element connected to the grounded-gate transistor of each unit circuit, and provides broadband amplification.
FIG. 1 is a circuit diagram of a conventional cascode distributed amplifier. The conventional cascode distributed amplifier in FIG. 1 is constructed by a plurality of unit circuits, each including a first transistor Q1 and a second transistor Q2, which are connected in parallel. Specifically, in the example shown in FIG. 1, seven unit circuits (or seven stages) are connected in parallel. An input signal IN is fed to an input terminal 10 at one end of an input-side transmission wire, and an amplified signal OUT generated by the distributed amplifier is supplied from an output terminal 12 at one end of an output-side transmission wire.
In FIG. 1, a resistor R1 is a termination resistor that is provided at the other end of the output-side transmission wire, and a resistor R2 is a termination resistor that is provided at the other end of the input-side transmission wire. Each of the rectangular blocks in FIG. 1 indicates an inductance component of one of the transmission wires or the unit circuits. The sign “∇” in FIG. 1 indicates that one end of the corresponding component such as a resistor, an inductor or a capacitor, is grounded.
In each of the plurality of unit circuits in the conventional cascode distributed amplifier in FIG. 1, the second transistor Q2 has a gate (called a grounded gate) which is grounded via an input capacitor Cgate. Further, an inductance component “lcg” is connected in series between the input capacitor Cgate and the gate of the second transistor Q2. The second transistor Q2 has a drain connected to the output-side transmission wire, and a source connected to a drain of the first transistor Q1. The first transistor Q1 has a gate which is connected to the input-side transmission wire. The first transistor Q1 has a source grounded, and a drain connected to the source of the second transistor Q2. Further, an inductance component “lsd” is connected between the source of the second transistor Q2 and the drain of the first transistor Q1.
In amplifying operation of the conventional cascode distributed amplifier in FIG. 1, the first transistor Q1 of each unit circuit provides amplification of the input signal IN which is sent to the input terminal 10. The band of the amplification (or the frequency characteristics) of the conventional cascode distributed amplifier depends on the total inductance value of the inductors “lsd” and “lcg” at the first and second transistors Q1 and Q2 and the capacitance value of the input capacitor Cgate at the gate of the second transistor Q2. The amplification gain of the conventional cascode distributed amplifier depends on the gain of the first transistor Q1 of each unit circuit.
In amplifying operation of the conventional cascode distributed amplifier in FIG. 1, the second transistor Q2 of each unit circuit provides generation of a negative-polarity resistance in a high frequency band. As the negative-polarity resistance allows the voltage gain to be increased (or providing a gain-raising characteristic), the conventional cascode distributed amplifier is effective in preventing the decreasing of the voltage gain in a high frequency band.
However, the conventional cascode distributed amplifier has the following problems.
FIG. 2 shows the frequency characteristics of the conventional cascode distributed amplifier in FIG. 1. In FIG. 2, the abscissas axis is the frequency axis (in GHz), and the ordinates axis is the gain axis (in dB). |S11| is the characteristic curve that indicates changes of a reflectance coefficient of the signal on the input-side transmission wire. |S22| is the characteristic curve that indicates changes of a reflectance coefficient of the signal on the output-side transmission wire. |S12| is the characteristic curve that indicates changes of an isolation of the output-side transmission signal with respect to the input-side transmission signal. |S21| is the characteristic curve that indicates changes of a gain coefficient (voltage gain) of the output signal to the input signal.
As shown in FIG. 2, the stability of the amplifying operation of the conventional cascode distributed amplifier deteriorates in the vicinity of the cut-off frequency of the ladder filter, which is near 50 GHz. Specifically, as indicated by the characteristic curve |S21| in FIG. 2, the voltage gain of the output signal to the input signal has a rapid increase (abnormal peaking) in the vicinity of the cutoff frequency of about 50 GHz. The peaking of the voltage gain occurs due to the negative-polarity resistance used in such a high frequency band. In other words, the negative-polarity resistance of the conventional cascode distributed amplifier is rapidly increased in the vicinity of the cut-off frequency, and the oscillation of the cascode distributed amplifier occurs at such frequency, which results in a rapid increase of the voltage gain of the output signal to the input signal. When the peaking of the voltage gain occurs, the eye pattern of the conventional cascode distributed amplifier deteriorates, and the possibility of a receiving error for the input signal is increased.
Further, in the conventional cascode distributed amplifier in FIG. 1, variations of the frequency characteristics of a subsequently connected circuit (for example, an LN modulator) due to changes of the manufacturing conditions thereof are not taken into consideration. If the frequency characteristics of the driver circuit for the LN modulator and the frequency characteristics of the LN modulator are not fitted to each other, each of the high level and the low level of the eye pattern has a certain width, and the error ratio of the signal transmitted by the transmission system is increased.